1. Field of the Invention
The present invention relates to a magneto-resistance effect element and a thin-film magnetic head which are preferable for use in hard disk drives.
2. Description of the Related Art
Hard disk drives employ a thin-film magnetic head having a magneto-resistance effect element (MR element) for reading magnetic signals. In recent years, efforts have been made to design hard disk drives for higher recording densities, and accordingly there are growing demands for thin-film magnetic heads, particularly magneto-resistance effect elements, which satisfy higher-sensitivity and higher-output requirements.
To meet the demands, there has been developed a magneto-resistance effect element including a spin-valve film (SV film) that has a structure wherein a nonmagnetic spacer layer is sandwiched between a pinned layer whose magnetization direction is fixed and a free layer whose magnetization direction is variable depending on an external magnetic field. The pinned layer and the free layer are formed as ferromagnetic layers. The pinned layer has its magnetization direction fixed by being disposed on an antiferromagnetic layer. Recently, there has been developed a synthetic SV film including a pinned layer that has a three-layer structure comprising a ferromagnetic layer, a nonmagnetic metal layer, and a ferromagnetic layer, rather than a single-layer structure having a ferromagnetic layer, so that a strong exchange coupling is given between the two ferromagnetic layers to effectively increase the exchange coupling force from the antiferromagnetic layer.
There has also been proposed a CPP (Current-Perpendicular-to-Plane) magneto-resistance effect element in which a sensing current flows perpendicularly to a layer surface for an increased output. It is desirable in the CCP magneto-resistance effect element that the ferromagnetic layer has a large polarizability. If the polarizability is large, then the rate of change of the magneto-resistance (also referred to as MR ratio), which serves as an indicator representative of the sensitivity of the magneto-resistance effect element, is large. Particularly, a TMR element employing a Heusler alloy (Co2MnAl, Co2MnSi, or Co2MnGe) which has a relatively high Curie temperature has a relatively high MR ratio at room temperature.
Japanese Patent Application Laid-Open No. 2003-218428 discloses a magneto-resistance effect element wherein at least one of the pinned and free layers has a ferromagnetic half-metal alloy layer. Japanese Patent Application Laid-Open No. 2003-218428 shows, as an example of the ferromagnetic half-metal alloy layer, a layer made of a full-Heusler alloy expressed by a composition formula X2YZ (where X represents an element selected from groups 3A to 2B of the periodic table, Y represents Mn, and Z represents one or more elements selected from Al, Si, Ga, In, Sn, Ti, and Pb).
As one of means for increasing the MR ratio of a magneto-resistance effect element, it has been proposed to insert an extremely thin oxide layer (NOL: Nano-oxide-layer) between a pinned layer and a free layer for thereby confining current path in a spin-valve film (Japanese Patent Application Laid-Open No. 2002-208744). According to the proposal, the flow of a sensing current is controlled to maximize the effect of a spin-dependent dispersion which the material has. Transaction of the Magnetic Society of Japan, Vol. 29, No. 9, pp. 869-877 (2005) discloses the recent data of a confined-current-path type CPP-GMR element. FIG. 5 of the document shows that the RA value that is resistance value per unit area of the element is 0.57 Ωμm2 and the MR ratio is 8.2%.
If a magneto-resistance effect element is applied to a thin-film magnetic head having a recording density in excess of 300 Gbpsi, then the S/N ratio is better as RA is lower and the MR ratio is higher. However, the confined-current-path type magneto-resistance effect element disclosed in Transactions of the Magnetic Society of Japan, Vol. 29, No. 9, pp. 869-877 (2005) has suffered a problem in that RA has to be large in order to obtain the effect according to the confining current path. Actually, according to FIG. 5 of the document, the MR ratio is about 4 through 5% in a practical low RA range (RA=0.2 through 0.3 Ωμm2). The inserted oxide layer gives rise to the problem of excessive oxidation of the pinned and free layers adjacent thereto.
For a magneto-resistance effect element employing a Heusler alloy, it is extremely important that the Heusler alloy have a particular crystalline structure (L21 structure or B2 structure) in order to achieve a high polarizability. In order for the Heusler alloy to have the particular crystalline structure, it is important that the proportions of the elements X, Y, Z of the Heusler alloy be represented virtually by a stoichiometric composition. The stoichiometric composition for the Heusler alloy is X:Y:Z=2:1:1 for a full-Heusler alloy and X:Y:Z=1:1:1 for a half-Heusler alloy.
If a full-Heusler alloy layer is actually provided adjacent to the spacer layer, then the proportions of the elements of the full-Heusler alloy layer are varied due to mutual diffusion of the layers, making it difficult to achieve the above particular crystalline structure. As a result, the polarizability of the pinned and free layers is not large as desired, and the use of the full-Heusler alloy is not effective-enough.